1. Field of the Invention
The present invention relates to an optical pickup that emits a beam to an optical disk and records and regenerates information signals to and from the optical disk.
2. Description of the Related Art
Optical disks, or optical recording media are used to store information signals or data including motion picture data, voice data, and computer data. The optical disks are mass-producible at low cost, and therefore, are widely used. Increasing requests for the optical disks are to improve the recording density and capacity thereof.
To improve the recording density of an optical disk, there are two approaches. One is to shorten the wavelength of light used to read data from the optical disk. The other is to increase the numerical aperture (NA) of an objective lens used to focus light on the optical disk.
When CDs (compact disks) were developed into DVDs (digital versatile disks or digital video disks), the wavelength was shortened from 780 nm to 650 nm and the objective-lens NA was improved from 0.45 to 0.60, thereby achieved a density improvement of about seven times from 650 MB to 4.7 GB (one side).
Recordable optical disks employ nearly the same wavelength and NA as those mentioned above, irrespective of their types such as a magneto-optical type and a phase change type.
A typical optical pickup presently used to read/write an optical disk employs a single objective lens formed from glass or resin. The objective lens has aspherical end surfaces to correct aberration. Lenses of this type are mass-producible by molding at low cost, and therefore, are widely used.
To further improve the recording density and capacity of an optical disk, a pickup employing a blue laser as well as an objective lens having a high NA must be developed.
An optical pickup employing a light source of 450 nm or shorter in wavelength and an objective lens of 0.7 or greater in NA must simultaneously correct axial chromatic aberration and spherical chromatic aberration. The axial chromatic aberration is a focal point variation due to a wavelength variation, and the spherical chromatic aberration is spherical aberration due to a wavelength variation. In this specification, the spherical chromatic aberration is called wavelength-error-based spherical aberration.
The reason why the objective lens having an NA of 0.7 or greater and employing a light source of 450 nm or shorter in wavelength necessitates the correction of axial chromatic aberration and wavelength-error-based spherical aberration will be explained.
First, the light of 450 nm in wavelength causes large dispersion by optical material such as glass of the objective lens, to produce large axial aberration and large spherical aberration.
Second, the increased NA of the objective lens increases refraction angles along the periphery of the lens. Even a small wavelength variation causes a large refraction angle change, to cause large spherical aberration.
The axial chromatic aberration and wavelength-error-based spherical aberration are each chromatic aberration. They, however, are caused by different reasons and have different characteristics.
The axial chromatic aberration is caused in an optical pickup by wavelength spread due to superimposed high frequencies applied to a laser diode, by a sudden wavelength variation due to a sudden power change at the laser diode during the recording of an optical disk, or by a wavelength error due to an individuality of the laser diode.
The axial chromatic aberration due to power change suddenly occurs in synchronization with a power change. This sudden change is difficult to follow by a focus servo mechanism that drives an objective lens of the optical pickup in a focusing direction. A range of wavelengths that must be coped with is about ±1 to ±2 nm. In the case of a laser diode employing superimposed high frequencies, it simultaneously emits beams of different wavelengths to a lens, and therefore, always causes focus errors in connection with wavelengths other than a reference wavelength.
If the optical pickup receives wavelengths spreading in a certain range or encounters a sudden wavelength variation, it will cause a focusing error due to axial chromatic aberration. This focusing error (defocusing) is severe, and therefore, must be corrected.
The wavelength-error-based spherical aberration is caused by wavelength variations due to the individuality of a laser diode and by changes in the temperature of the laser diode.
The wavelength-error-based spherical aberration is stable or changes relatively slowly, and a range of wavelengths that must be coped with is about ±5 to ±10 nm.
It has been known heretofore that a diffraction lens having chromatic aberration characteristics of which polarity is reverse to that of the refraction lens is useful for the correction of the chromatic aberration. It can be said that, most simply, if a lens system setting lens power thereof at zero is placed before the objective lens, then the correction of the chromatic aberration can be executed. In this case, this lens system is formed by combining a diffraction lens made into a convex lens and a refraction type concave lens of which absolute value of a focal distance is equal to that of the diffraction lens.
Here, the diffraction lens is a lens having a blaze structure formed on a flat plate, which is designed to collect axial beams without any aberration in a designed wavelength at a designed reference focal distance, and has high diffraction efficiency in the designed wavelength.
However, in the case where a single objective lens is applied to a light source having a high numerical aperture and a short wavelength, for example, such as the numerical aperture of 0.7 or greater and the wavelength of 450 nm of shorter, the spherical aberration due to the wavelength error generated in the case where the wavelength is shifted from the designed reference wavelength is increased, thus causing a problem that a sufficient correction cannot be performed by means of a simple diffraction lens.
Meanwhile, Japanese patent Laid-Open Publication No. 6-82725 suggests a designing method for a diffraction lens, in which the chromatic aberration generated in the objective lens is completely corrected. The contents of the above are described in detail in “Kaisetsu kogaku Soshi Nyumon”, Optronics, Co., Ltd., p. 94 by the inventors of this publication.
However, according to this method, though the spherical aberration due to the wavelength error can be corrected, there is a problem that a large aberration appears by a lens shift caused by the tracking operation of the objective lens when a correction lens is placed on the fixed portion of the optical system. Specifically, the optical axes of the objective lens driven by the tracking operation and another optical system of the fixed portion are shifted from each other, and thus an aberration containing a comatic aberration as a main component is caused, which will then adversely affect the recording/reproduction of the optical disk significantly.
Although this problem can be avoided if the correction lens is moved integrally with the objective lens, a movable portion of an actuator for driving these objective and correction lenses is increased in weight, and the frequency characteristics of the actuator are lowered, causing a problem that a necessary band cannot be secured.